This invention relates to a portable power supply apparatus, which houses a charging circuit and a discharging circuit, and to the method of power supply from this apparatus to electrical equipment.
In general, a battery pack which houses rechargeable batteries and which is detachable from electrical equipment is used as the power source for portable electrical equipment such as a video camera. When the battery pack has discharged, it is removed from the electrical equipment, attached to a battery charger separate from the electrical equipment, and charged for reuse.
Used in this manner, the battery pack containing rechargeable batteries is quite convenient. However, as the power source for electrical equipment, the battery pack must first be attached to the electrical equipment, removed from the electrical equipment when is has discharged, and attached to a battery charger for recharging. This operation is troublesome.
As one method of improving ease of operation, Japanese Non-examined Patent Publication No. 6-5311 issued Jan. 14, 1994 discloses a battery pack containing a charging circuit.
Typically, since one battery pack is attached to one electrical equipment unit and the usable time is short, spare battery packs are carried for long duration use in the field. Here, it is necessary to replace battery packs with spare battery packs whenever those attached to the electrical equipment become discharged. In this respect, a battery pack containing a charging circuit requires the same replacement procedure.
Thus the first object of the present invention is to provide a power supply apparatus which can be used for long duration without requiring battery pack detachment and reattachment, and which improves ease of operation by reducing detachment and attachment operations as much as possible.
The inventor and associates have developed a portable power supply apparatus on which a plurality of battery packs are mounted. Since this power supply apparatus delivers power to electrical equipment from a plurality of battery packs, it has the advantage that the usable time of the electrical equipment is extended compared to electrical equipment with a single battery pack attached. This configuration of power supply apparatus uses the plurality of battery packs in order by switching from one to the next. When the first battery pack output voltage drops to a final voltage level, the apparatus switches to the next battery pack to be used. The apparatus continues to switch from the battery pack in use to the next battery pack whenever output voltage drops to the final voltage level. Since the discharging battery pack is switched to a fully charged battery pack when its output voltage drops to the final voltage level, the output voltage of the power supply apparatus becomes greater than the final voltage level.
Incidentally, electrical equipment which use batteries as their power source can display remaining battery capacity to keep track of battery use. Remaining battery capacity is displayed by sensing battery voltage. The remaining battery capacity display function operates normally when the electrical equipment is powered by its internal batteries. However, when a power supply apparatus is connected to the electrical equipment and power is delivered from the power supply apparatus battery packs to the electrical equipment, the remaining battery capacity display function fails to operate properly. This is particularly noticeable, for example, when a battery pack containing lithium-ion rechargeable batteries, which reduce output voltage in proportion to remaining battery capacity, are attached to the power supply apparatus. A power supply apparatus, which has a battery pack containing lithium-ion rechargeable batteries attached, houses a DC--DC converter. The DC--DC converter is a stabilization circuit which stabilizes power supply apparatus output voltage by increasing output voltage when battery pack voltage drops due to discharge. When battery pack output voltage drops to the final voltage level, the power supply apparatus switches to use the next battery pack. However, although battery pack voltage drops, power supply apparatus output voltage does not drop.
Electrical equipment supplied by a constant output voltage from the power supply apparatus cannot display remaining battery capacity by sensing voltage. This has the disadvantage that remaining usable time is unknown and the equipment may become unusable without warning. Thus the second object of the present invention is to provide a power supply apparatus to provide power to electrical equipment from a plurality of battery packs and its method of power supply which avoids this disadvantage by supplying a stable output from the power supply apparatus to the electrical equipment and by allowing display of remaining battery capacity at the electrical equipment.
A power supply apparatus which provides power to electrical equipment from a plurality of battery packs has the further advantage that since the electrical equipment is driven by a plurality of battery packs, the usable time duration of the electrical equipment is lengthened considerably compared to electrical equipment containing a single battery pack. However, since power is not delivered directly from battery packs to the electrical equipment, this method of power delivery from a power supply apparatus has the disadvantage that power losses results from lead, contact terminal, and other voltage losses along the way from the battery packs. A power supply apparatus which houses a DC--DC converter can deliver power while compensating for voltage losses from the battery packs to the electrical equipment. A power supply apparatus with a simple circuit structure which does not house a DC--DC converter cannot deliver power to electrical equipment compensated for voltage losses. For this reason such an apparatus delivers voltage to electrical equipment which is lower than the output voltage of the battery packs.
Incidentally, if the battery pack voltage of an electrical equipment unit drops below a cut-off voltage, the power switch is forcibly turned off. This is to prevent over-discharge and performance degradation of the battery pack. The cut-off voltage where the electrical equipment turns the power switch off is set lower than the final voltage level where the power supply apparatus switches battery packs. If the cut-off voltage was set higher than the final voltage level, the electrical equipment power switch would be turned off before the power supply apparatus could switch battery packs and a plurality of battery packs could not be used.
In this manner, the fact that the power supply apparatus final voltage level must be set higher than the electrical equipment cut-off voltage combined with the fact that the battery pack output voltage is actually lower than the electrical equipment supply voltage means that the real capacity of the power supply apparatus battery packs is reduced. The third object of the present invention is to overcome this disadvantage. Thus the third object of the present invention is to provide a power supply apparatus to provide power to electrical equipment from a plurality of battery packs and it method of power supply which effectively uses the attached battery packs to allow long duration use of electrical equipment.
The above and further objects and features of the invention will more fully be apparent from the following detailed description with accompanying drawings.